Multiple-input multiple-output (MIMO) wireless systems exploiting multiple antennas as both transmitters and receivers have attracted increasing interest due to their potential for increased capacity in rich multipath environments. Such systems can be used to enable enhanced communication performance (i.e. improved signal quality and reliability) by use of multi-path propagation without additional spectrum requirements. This has been a well-known and well-used solution to achieve high data rate communications in relation to 2G and 3G communication standards. For indoor wireless applications such as router devices, external dipole and monopole antennas are widely used. In this instance, high-gain, omni-directional dipole arrays and collinear antennas are most popular. However, very few portable devices with MIMO capability are available in the marketplace. The main reason for this is that, when gathering several radiators in a portable device, the small allocated space for the antenna limits the ability to provide adequate isolation between each radiator.
A reconfigurable MIMO antenna is known from WO 2012/072969 (the content of which is incorporated into the present disclosure by reference). An embodiment is described in which the antenna comprises a balanced antenna located at a first end of a PCB and a two-port chassis-antenna located at an opposite second end of the PCB. However, in certain applications this configuration may not be ideal or even practical since it requires two separate areas in which to locate each antenna. However, as mentioned above this spacing was chosen to provide adequate isolation between each antenna structure.
Another reconfigurable antenna is known from WO 2014/020302 (the content of which is incorporated into the present disclosure by reference). This antenna comprises a balanced antenna and an unbalanced antenna mounted on a supporting PCB substrate, with both the balanced antenna and the unbalanced antenna located at the same end of the substrate. The antenna may be configured as a chassis antenna for use in a portable device and may be configured for MIMO applications. In one embodiment of the antenna of WO 2014/020302, there is provided a floating groundplane connected to the balanced antenna. The floating groundplane is constituted by a rectangular metal patch located on a first surface of the substrate, centrally below feed lines provided on the first surface to feed the balanced and unbalanced antennas. A first matching circuit configured to excite the arms of the balanced antenna is located on the floating groundplane. The unbalanced antenna is mounted on a second surface of the substrate, opposed to the first surface, and is connected to a second matching circuit mounted on the PCB substrate. In another embodiment, the floating groundplane may be incorporated in one arm of the balanced antenna, thereby saving space on the PCB substrate. Each matching circuit is coupled to a signal port, and the antenna as disclosed therefore provides only two ports.
Certain handset and other portable device manufacturers, however, require an antenna with more than two ports. A particular problem with multi-port antennas, particularly when there more than just two ports, is how to obtain sufficient isolation between the ports.